1. Field of the Invention
This invention generally relates to an elastic boundary wave device and a method of manufacturing the same, and more particularly, to an elastic boundary wave device employed for a filter or oscillator on a television set, cellular phone, or PHS (Personal Handyphone System), and a method of manufacturing the same.
2. Description of the Related Art
Conventionally, a surface acoustic wave device (hereinafter referred to as SAW device) has been known well as one of the devices that utilize elastic waves. The SAW device is employed in various types of circuits on devices that send and receive wireless signals, for instance, in a frequency range of 45 MHz to 2 GHz. Examples of the above-mentioned various types of circuits are a bandpass filter for transmission and reception, local oscillation filter, antenna duplexer, IF filter, and FM modulator.
The SAW device utilizes elastic waves that travel on an interface between a solid surface and vacuum or gas, that is, the SAW device utilizes the elastic waves that travel on the solid surface. Therefore, a piezoelectric substrate is required to have a free surface so as to serve as a propagation medium. This does not allow the SAW device to be covered with plastic mold, which is generally used for packaging semiconductor devices. In order to obtain the free surface, it is necessary to have a hollow portion inside the package. However, the use of the hollow portion in the package may cause a problem in that the SAW device is relatively expensive and large-scaled.
In contrast, in recent years, the boundary wave devices have been studied. The boundary wave devices utilize the elastic wave that travels on the boundary surface between solids, as disclosed in International Publication Number WO 98/51011 (hereinafter referred to as Document 1) and Yamashita et al., “Highly Piezoelectric Boundary Waves in Si/SiO2/LiNbO3 Structure”, Japan Society for the Promotion of Science, The 150th Committee of Elastic Surface Wave Device Technique, The 53rd Study Material, Jul. 11, 1997, pp. 19-24 (hereinafter referred to as Document 2). The boundary wave device is an elastic wave device that utilizes the elastic wave traveling on the interface between two solids that are in contact with each other. The elastic wave of the boundary wave device propagates in the vicinity of the boundary of two substrates. Therefore, the boundary wave device has no limitation on the propagation surface. This is different from the SAW device, because the piezoelectric substrate of the SAW device is required to have a free surface. The boundary wave device has an advantage in that the size and cost of the package can be reduced easily, and in addition, the boundary wave device is able to obtain capabilities equivalent to those of the SAW devices.
The elastic boundary wave devices disclosed in Documents 1 and 2 will be described, with reference to the accompanying drawings. FIG. 1 is an exploded perspective view of an elastic boundary wave device 900. FIG. 2 is a cross-sectional view taken along a line A-A′ shown in FIG. 1. Referring to FIGS. 1 and 2, the elastic boundary wave device 900 includes a first substrate 902 which is a piezoelectric substrate, comb-like electrodes 903 (interdigital transducer, hereinafter referred to as IDT) which are arranged on the first substrate 902, a second substrate 906 which is a silicon(Si)-based substrate. The first substrate 902 and the second substrate 906 are respectively joined to a dielectric film 905 that forms an interposer. The dielectric film 905 covers the IDTs 903 and has a smoothed surface 904. The second substrate 906 is joined to the smoothed surface 904.
As shown in FIG. 1, a piezoelectric substrate made of LiNbO3 is used for the first substrate 902. A silicon substrate is used for the second substrate 906. All of the above-mentioned substrates have negative temperature coefficients of phase velocity (hereinafter referred to as TCV). Document 2 discloses that the SiO2 film having a positive TCV is employed for the dielectric film 905. Document 2 also discloses that the joining interface has a high electromechanical coupling coefficient and there exists boundary waves having zero-temperature characteristics when the interface is defined by an SiO2 film that is interposed between the LiNbO3 substrate and the silicon substrate and has a TCV opposite to those of these substrates.
Document 1 has proposed a manufacturing process of the above-mentioned boundary wave device 900, which will be described, with reference to FIGS. 3A through 3D and 4.
Referring to FIG. 3A, a metal film 903a made of, for example, aluminum (Al) is deposited on a first main surface (herein after referred to as upper surface) of the first substrate 902. This is performed by, for example, sputtering. Next, referring to FIG. 3B, the metal film 903a is processed by etching in order to define multiple IDTs 903, interconnection patterns and terminal patterns (hereinafter referred to as metal patterns), which are connected to the multiple IDTs 903. Referring to FIG. 3C, a dielectric film 905a made of, for example, SiO2 is provided on the surface of the first substrate 902 on which the metal patterns have been arranged. This is performed by, for example, sputtering. Then, referring to FIG. 3D, a dielectric film 905a having a smoothed surface 904 is formed by polishing the surface of the dielectric film 905a. 
Next, referring to FIG. 4, surface processing is performed on the smoothed surface 904 and a lower surface of the second substrate 906 respectively to be joined together. This is to hydrate the above-mentioned surfaces with ammonia water, for example. Then, the hydrated smoothed surface 904 and lower surface of the second substrate 906 are brought into contact, and are heated at approximately 300° C. for one to two hours. Thus, OH groups on the respectively joined surfaces form a chemical union, while H2O is liberated, so that the dielectric film 905 and the second substrate 906 can be joined together, although the dielectric film 905 and the second substrate 906 are heterogeneous substrates.
As described above, it is necessary to form a relatively thick dielectric film of approximately 2.4 μm in order to produce the boundary wave device having the zero-temperature characteristic, even in a relatively high frequency range of around 1 GHz. However, in the case where the above-mentioned dielectric film is formed by sputtering, for example, generally, the first substrate 902 may be warped significantly due to internal stress of the dielectric film. Therefore, there is another problem in that joining strength cannot be maintained evenly on the entire joined surfaces of the dielectric film and the second substrate 906.
Besides, when the first substrate 902 and the second substrate 906 are joined together, after hydrating the both surfaces of the dielectric film 905 formed on the first substrate 902 and the second substrate 906, it is necessary to heat the both surfaces at approximately 300° C. for one to two hours. However, in the case where the substrates are joined with the above-mentioned thermal treatment and are cooled down to room temperature, there is yet another problem in that the joined substrates are warped because of the difference in thermal expansion coefficient between the first substrate 902 and the second substrate 906. Further, in the case where a process is performed on the above-mentioned warped substrates so as to expose an electrode or the like, there is a further problem in that uniform exposure cannot be achieved with a light exposure process or the like.